Pigment particles are used in a variety of applications. For example, dielectric pigment particles suspended in a fluid medium are used in electrophoretic image displays (EPIDs) and electrostatic printers, both of which operate according to the electrophoretic effect. The electrophoretic effect is well known in the art as evidenced by the many patents and articles which describe the effect. As one of ordinary skill in the art will recognize, the electrophoretic effect operates on the principle that pigment particles suspended in the medium can be electrically charged and thereby caused to migrate through the medium to an electrode of opposite charge. For an example of devices using the electrophoretic effect, reference is made to U.S. Pat. No. 4,732,830, issued to Frank J. DiSanto et al., on Mar. 22, 1988, entitled ELECTROPHORETIC DISPLAY PANELS AND ASSOCIATED METHODS, and assigned to Copytele, Inc., the assignee herein.
In typical EPIDs, the dielectric pigment particles are suspended in a fluid medium that is either clear or of an optically-contrasting color as compared with the dielectric pigment particles. To effect the greatest optical contrast between the pigment particles and the fluid suspension medium, it is desirable to have either light-colored pigment particles suspended in a dark medium or dark-colored pigment particles, preferably black, suspended in a backlighted clear medium. A variety of pigment particle and dispersion medium compositions are known in the art. See, for example, U.S. Pat. No. 4,298,444, issued to K. Muller, et al., on Nov. 3, 1981, entitled ELECTROPHORETIC DISPLAY.
It has been found difficult to produce black and other very dark-colored pigment particles that are dielectric, of uniform size, and have a density matching that of a common suspension medium. As a result, EPEDs commonly use readily-manufactured light colored dielectric pigment particles suspended in fluid medium which contain a dark color dye. Such EPIDs are exemplified in U.S. Pat. No. 4,655,897 to DiSanto et al.; U.S. Pat. No. 4,093,534 to Carter et al.; U.S. Pat. No. 4,298,448 to Muller et al.; and U.S. Pat. No. 4,285,801 to Chiang. The use of a yellow pigment particle is disclosed in the following patents, all of which issued to Frederic E. Schubert and are assigned to Copytele, Inc., the assignee herein: U.S. Pat. No. 5,380,362, issued Jan. 10, 1995, entitled SUSPENSION FOR USE IN ELECTROPHORETIC IMAGE DISPLAY SYSTEMS; U.S. Pat. No. 5,403,518, issued Apr. 4, 1995, entitled FORMULATIONS FOR IMPROVED ELECTROPHORETIC DISPLAY SUSPENSIONS AND RELATED METHODS; and U.S. Pat. No. 5,411,656, issued May 2, 1995, entitled GAS ABSORPTION ADDITIVES FOR ELECTROPHORETIC SUSPENSIONS.
The selection of the dielectric pigment particles used in the EPID (electrophoretic particles) is very important in determining performance of the EPID and the quality of the image produced. Ideally, the electrophoretic particles should have an optimum charge-to-mass ratio, which is dependent upon the particle size and surface charge; the optimum charge-to-mass ratio is desirable to obtain good electrostatic deposition at high velocity as well as rapid reversal of particle motion when voltages change.
There are advantages to polymerically coating the pigment particles used in EPIDs or other electrophoretic-effect devices. A polymer coating applied to the pigment particles can have the effect of enhancing the ability of the pigment particles to scatter light. When polymer-coated pigment particles impinge upon a transparent screen electrode (thereby displacing the fluid medium from the screen), a brighter color and sharper image is produced (as compared with when uncoated pigment particles are used). Additionally, it naturally is desirable to use pigment particles that are stable and resistant to interaction with the fluid suspension medium to improve the efficiency and lifetime of the EPID; the fluid suspension medium may comprise, for example, aromatic or aliphatic solvents, including benzenes, toluenes, hydrocarbon oil, nonane, decane, or the like, which may react with some typical pigment particles used in EPIDs. Polymer-coated pigment particles can produce a harder and more solvent-resistant composite when compared to uncoated pigment particles.
Furthermore, polymer-coated pigment particles can be less apt to adhere to surfaces within the electrophoretic cell. Over recent years, attention has been directed to dispersion stabilization by way of adsorbed polymers on particle surfaces. See, for example, an article by P. Murau and B Singer, appearing in Vol 49, No. 9, of the Journal of Applied Physics (1978), entitled "The Understanding and Elimination of Some Suspension Instabilities in an Electrophoretic Display." See also U.S. Pat. No. 5,403,518, issued to Schubert, referenced above, and U.S. Pat. No. 4,285,801, issued to Anne A. Chiang on Aug. 25, 1981, entitled ELECTROPHORETIC DISPLAY COMPOSITION.
It has been discovered that fluoro polymers are advantageous for use as pigment particle coatings, as they have low critical surface tensions which tend to produce anti-stick properties. See, for example, an article by M. Hudlicry and A. E. Pavlath, appearing in Vol. 187, page 983, of ACS Monograph (1995), entitled "Properties of Fluorinated Compounds II." The lower surface tensions of perfluorinated polymers, when used as a pigment particle coatings, leads to less interaction between the pigment particle surfaces and the solvents comprising the fluid suspension medium in which the pigment particles are dispersed. The stabilizing polymer molecule hydrocarbon chains are more apt to extend out into the solvent and not sit on the particle surface because of its anti-static character. This enhances the efficiency of the stabilizing mechanism.
There are many different methods for preparing polymer-coated particles. For example, thirty different methods are reported in an article by R. E. Sparks, et al., edited by M. A. El-Nokaly et. al., Polymeric Delivery Systems ACS Symposium Series #520, Washington D.C. (1993). All of these methods produce a three component mixture comprising polymer-coated particles, uncoated particles and pure polymer particles (polymer particles having no pigment particle core for example).
Accordingly, a need exists for an improved method of manufacturing polymer-coated pigment particles which yields substantially no uncoated pigment particles and substantially no polymer particles having no pigment particle core.